The process of evaporation of water from the body of plants is called transpiration. It mainly occurs from the surface of leave of the plants. Under the surface of leaf there is pores called stomata which is called as the site of transpiration. It is the passage of oxygen, carbon dioxide and water in and out the stomatal opening. Stomata opens when the sunlight strikes the leaf surface at the time of dusk and it closes when there is no incoming light like during the night. Stomata opening and closing is regulated by guard cells. Guard cells regulate the turgor pressure inside the plants. The increase in osmotic pressure of the guard cells as compared to the surrounding cells tends to open the stomata and thus the excess water is transpired through the cell into the environment. During evening time, osmotic pressure of the guard cells decrease as compared to the surrounding cells, the stomata closes, which results in no loss of water to the environment. Reason behind the increase and decrease of turgor or osmotic pressure in the guard cells is due to uptake of potassium ions.